White-emitting phosphor components for a CRT data display screen are selected according to certain characteristics, such as: color temperature, brightness and rate of decay or persistence. Due to the nature of data display applications, which may often portray substantially static imagery with periodic updating, it is desirable to have luminescent emission whereof substantially continuous and concentrated viewing will produce a minimum of eye fatigue.
In display tubes operated in accordance with present television standards of 525 horizontal scan lines per frame, each frame of information is scanned twice by the exciting electron beam. This rapidly moving beam travels from left to right and from top to bottom across the screen of the tube at a uniform sweep pitched at a slightly downward slope. At the end of each horizontal line, the beam is returned, at a greater velocity, to the left side of the screen by an action known as horizontal retrace. Simultaneously to the horizontal sweep and retrace, each scan of the beam is pulled downward by the vertical sweep. When the horizontal sweep reaches the bottom of the screen, the vertical retrace returns the beam back to the top of the screen for the next scan. In this manner, each vertical sweep of the screen is accompanied by 262.5 horizontal sweeps and, as such, forms a "field" which contains only half of the display imagery of a complete frame. The first vertical sweep of each frame, known as the "odd field", fills the odd-numbered lines of the 525 in order from top to bottom, while the related second vertical sweep or "even field" fills in the even-numbered lines of the frame. The combining of these "odd and even fields", each comprising 262.5 lines, is known as interlaced scanning.
By conventional television standards, each vertical sweep or field is accomplished in 1/60 of a second or 16.66+ milliseconds. This frequency of field scanning is designated as a repeat or refresh rate of 60 Hertz (Hz). Since two fields constitute a complete 525-line frame of display imagery, the frequency rate of frame production is thirty per second, or once every 33.33+ milliseconds.
Cathodoluminescent phosphors employed in CRT screens exhibit two related luminescent characteristics: fluorescence and phosphorescence. Fluorescence is the luminescent build-up or emission of light released from the phosphor during the time of electron beam excitation. The duration of phosphorescence, or rate of decay of afterglow, is denoted as persistence. This is usually expressed as a measurement of time required for the phosphorescence to reduce or decay to a ten percent level of steady state fluorescent brightness.
Phosphors selected for utilization in data display CRT's operated in accordance with the aforedescribed interlaced scanning at a 60 Hz rate of refresh, exhibit decay persistences substantially within the range of 500 microseconds (.mu.sec) to one millisecond (M sec), and as such are generally classified as having medium to medium-short persistences. These phosphors, when excited under the aforenoted operating conditions in conjunction with the visual persistence acuity of the human eye, provide a display which is interpreted by the observer as flicker-free imagery.
It has been found that a marked reduction in terminal equipment costs can be realized by utilizing less expensive components and circuitry to provide a scan refresh rate less than the conventional 60 Hz. But, when an economical rate of field refresh, for example in the order of 20 to 40 Hz, is employed in exciting CRT screens comprised of conventional medium persistence phosphors, noticeable flicker or brightness variation becomes evident in the screen display. This produces a deleterious viewing situation which is both distracting and fatiguing for the operator of the data display terminal.